Field of the Invention
The present invention relates to an organic light-emitting diode display having an aperture ratio improved by forming a storage capacitor in an emission area using a transparent conductive material, and to a method for manufacturing the same. In addition, the present invention relates to an organic light-emitting diode display and a method for manufacturing the same for simplifying a manufacturing process by reducing the number of mask processes.
Discussion of the Related Art
Recently, a variety of flat panel displays having reduced weight and volume, compared to cathode ray tubes, has been developed. Such flat panel displays include liquid crystal displays (LCDs), field emission displays (FEDs), plasma display panels (PDPs), electroluminescent devices (ELs) and the like.
ELs are classified into an inorganic EL and an organic light-emitting diode display and are self-emissive devices having the advantages of high response speed, luminous efficiency and brightness and wide viewing angle.
FIG. 1 illustrates a structure of an organic light-emitting diode according to the related art. As show in FIG. 1, the organic light-emitting diode includes an organic electroluminescent compound layer, a cathode and an anode opposite to each other and having the organic electroluminescent compound layer interposed therebetween. The organic electroluminescent compound layer includes a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL) and an electron injection layer (EIL).
The organic light-emitting diode emits light according to energy from excitons generated through a process in which holes and electrons injected to form the anode and the cathode are recombined in the EML. An organic light-emitting diode display displays images by electrically controlling the quantity of light generated in the EML of the organic light-emitting diode as shown in FIG. 1.
Organic light-emitting diode displays (OLEDDs) using the characteristics of the organic light-emitting diode which is an electroluminescent device are classified into a passive matrix type organic light-emitting diode display (PMOLED) and an active matrix type organic light-emitting diode display (AMOLED).
The AMOLED displays images by controlling current flowing through organic light-emitting diodes using a thin film transistor (referred to as TFT hereinafter).
FIG. 2 is an equivalent circuit diagram illustrating a structure of one pixel of an organic light-emitting diode display according to the related art, FIG. 3 is a plan view of the structure of one pixel of the organic light-emitting diode display according to the related art and FIG. 4 is a cross-sectional view illustrating the structure of the conventional organic light-emitting diode display, taken along line I-I′ of FIG. 3.
Referring to FIGS. 2 and 3, an AMOLED includes a switching TFT (TFT) ST, a driving TFT DT connected to the switching TFT ST and an organic light-emitting diode OLED in contact with the driving TFT DT.
The switching TFT ST is formed at an intersection of a scan line SL and a data line DL and serves to select a pixel. The switching TFT ST includes a gate electrode SG, a semiconductor layer SA, a source electrode SS and a drain electrode SD. The driving TFT DT drives an organic light-emitting diode OLED of a pixel selected by the switching TFT ST. The driving TFT DT includes a gate electrode DG connected to the drain electrode SD of the switching TFT ST, a semiconductor layer DA, a source electrode DS connected to a driving current line VDD and a drain electrode DD. The drain electrode DD of the driving TFT DT is connected to an anode ANO of the organic light-emitting diode OLED.
More specifically, referring to FIG. 4, the gate electrodes SG and DG of the switching TFT ST and the driving TFT DT are formed on a substrate SUB of the AMOLED. A gate insulating layer GI is formed on the gate electrodes SG and DG. The semiconductor layers SA and DA are formed on portions of the gate insulating layer GI, which correspond to the gate electrodes SG and DG. The source electrode SS and the drain electrode SD are formed on the semiconductor layer SA, opposite to each other having a predetermined gap provided therebetween. The source electrode DS and the drain electrode DD are formed on the semiconductor layer DA, opposite to each other having a predetermined gap provided therebetween. The drain electrode SD of the switching TFT ST is connected to the gate electrode DG of the driving TFT DT via a contact hole formed in the gate insulating layer GI. A passivation layer PAS is formed on the overall surface of the substrate so as to cover the switching TFT ST and the driving TFT DT having the aforementioned structure.
When the semiconductor layers SA and DA are formed of an oxide semiconductor material, a high resolution and fast driving speed can be achieved in a large TFT substrate having large charging capacity due to the oxide semiconductor's high mobility. The oxide semiconductor material layers may further include etch stoppers SE and DE for protecting the surfaces thereof from an etchant in order to ensure device stability. Specifically, the etch stoppers SE and DE are formed so as to prevent the semiconductor layers SA and DA from being back-etched due to an etchant contacting the exposed surfaces of the semiconductor layers SA and DA, which correspond to the gaps between the source electrodes SS and DS and the drain electrodes SD and DD.
A color filter CF is formed in a region corresponding to the anode ANO which will be formed later. The color filter CF is preferably formed to occupy a wide area if possible. For example, the color filter CF is formed such that the color filter CF is superposed on a wide area including the data line DL, driving current line VDD and scan line SL. The substrate on which the color filter CF has been formed has an uneven surface and many stepped portions since a lot of components have been formed thereon. Accordingly, an overcoat layer OC is formed on the overall surface of the substrate in order to planarize the surface of the substrate.
Subsequently, the anode ANO of the OLED is formed on the overcoat layer OC. Here, the anode ANO is connected to the drain electrode DD of the driving TFT DT via a contact hole formed in the overcoat layer OC and the passivation layer PAS.
A bank pattern BN for defining a pixel region is formed on the switching TFT ST, the driving TFT DT and the interconnection lines DL, SL and VDD formed on the substrate on which the anode ANO is formed.
The anode ANO exposed through the bank pattern BN becomes an emission area. An organic emission layer OLE and a cathode layer CAT are sequentially formed on the anode ANO exposed through the bank pattern BN. When the organic emission layer OLE is formed of an organic material emitting white light, the organic emission layer OLE expresses a color assigned to each pixel according to the color filter CF located under the organic emission layer OLE. The organic light-emitting diode display having the structure as shown in FIG. 4 is a bottom emission display which emits light downwardly.
In such a bottom emission organic light-emitting diode display, a storage capacitor STG is formed in a space in which the anode ANO is superposed on the gate electrode DG of the driving TFT DT. The organic light-emitting diode display displays image information by driving organic light-emitting diodes. Here, a considerably large amount of energy is necessary to drive the organic light-emitting diodes. Accordingly, a large-capacity storage capacitor is needed in order to correctly display image information having rapidly changing data values, such as video.
To secure a storage capacitor having sufficient capacity, a storage capacitor electrode needs to have a sufficiently large area. In the bottom emission organic light-emitting diode display, a light emitting area, that is, an aperture ratio, decreases as the storage capacitor area increases. In a top emission organic light-emitting diode display, the storage capacitor can be formed under the emission area and thus the aperture ratio does not decrease even when a large-area storage capacitor is designed. However, the area of the storage capacitor is directly related to aperture ratio decrease in the bottom emission organic light-emitting diode display.
To manufacture such an organic light-emitting diode display, a photolithography process using a photo-mask is performed multiple times. Each mask process includes cleaning, exposure, development, etching and the like. When the number of mask processes increases, time and costs for manufacturing the organic light-emitting diode display and a defect generation rate increase, decreasing production yield. Accordingly, it is necessary to reduce the number of mask processes in order to decrease manufacturing costs and improve production yield and production efficiency.